Cytosynalin: a Mr 35,000 cytoskeleton-interacting and calmodulin-binding protein.

A Mr 35,000 calmodulin-binding protein, which also possesses the ability to interact with a number of cytoskeletal elements, was purified from bovine synaptosomal membranes, using hydroxylapatite and phosphocellulose column chromatography. We call this protein "cytosynalin," which is derived from a combination of the words "cytoskeleton" and "synapsis." Indirect immunofluorescent staining of cytosynalin revealed that it is distributed throughout neural cells including in dendrites, varicosities, and growth cones. In 3T3 cells, cytosynalin is located on the inner surface of the plasma membrane. The staining pattern of cytosynalin observed with anti-cytosynalin antibodies was similar to that observed with antibodies against calspectin (nonerythroid spectrin or fodrin). Cytosynalin binds to calspectin and to tubulin and actin filaments, as determined by binding assays, turbidity measurements, low-shear viscometry, and electron microscopy. Furthermore, after heating at 90 degrees C for 5 min, cytosynalin retains its binding activity toward calmodulin and cytoskeletal elements. These results lead to the conclusion that cytosynalin is colocalized with calspectin at the inner surface of the plasma membrane, probably in association with other cytoskeletal elements.